Updates to overview, half of bulk import.

Signed-off-by: Christophe Taton <taton@wibidata.com>
KijiMR user guide : work in progress.

 - a lot more details in code snippets.
 - command-line examples

Still missing:
 - section on HFiles
 - update kiji transform
 - details on the testing framework
 - details on the input/output formats
 - check the KVstore section